Quartz glass elements

ABSTRACT

QUARTZ GLASS ELEMENT, SUCH AS A DIFFUSION TUBE USEFUL IN THE PRODUCTION OF SEMICONDUCTOR ELEMENTS, CAPABLE OF FORMING ON OUTER LAYER OF UNIFORMLY FINE CRYSTALLINE SILICA SUCH AS CRISTOBALITE OR TRIDYMITE WHEN HEATED TO A TEMPERATURE AT WHICH CRYSTALLINE SILICA FORMS CONTAINING CRYSTALLIZATION PROMOTING NUCLEI HAVING A RATE OF DIFFUSION IN QUARTZ GLASS LESS THAN THAT OF SODIUM AT ELEVATED TEMPERATURES. SUCH NUCLEI ARE PREFERABLY PRESENT IN THE OUTER   HALF OF THE ELEMENT WALL. WHEN THE QUARTZ GLASS ELEMENT IS EXPOSED TO ELEVATED TEMPERATURES, THE NUCLEI PROMOTES THE FORMATION OF THE OUTER LAYER OF UNIFORMLY FINE CRYSTALLINE SILICA WHICH IMPARTS THERMAL DIMENSIONAL STABILITY FOR EXTENDED PERIODS OF USE AT ELEVATED TEMPERATURES.

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INVENJ'ORS PETER BAUMLER GERHARD HOFER TASSILO KORNER HEINRICH MOHNKARlzHElNZ RAU KARL o. SEILER Y FRITZ SIMMAT BURGESS. DINKLAGE a. SPRUNGATTORNEYS.

United States Fatent 6 3,776,809 QUARTZ GLASS ELEMENTS PeterBiiumlen'Dorniglieim, Gerhard Htifer, Bruchkobel, Tassilo Kiirner,Erlensee, Heinrich Mohn, Hailer, Karl Seiler, Hauau-Hohe Tanne, FritzSimmat, Meerholz, and Karlheinz Rau, Hanan, Germany, assignors toHeraeus-Schott Quarzschmelze GmbH, Hanan, Germany a Continuation-impartof abandoned applications Ser. No. 793,775, Jan. 24, 1969, and Ser. No.810,713, Mar. 26, 1969. This application July 28, 1971, Ser. No. 166,844

Claims priority, application Germany, Feb. 22, 1968,

P 16 96 061.5; Mar. 30, 1968,P 17 71 077.9, H 62,286

Int. Cl. B32b 7/02, 17/06; C03c 3/06 U.S. Cl. 161-164 10 Claims ABSTRACTOF THE DISCLOSURE Quartz glass element, such as a diffusion tube usefulin the production of semiconductor elements, capable of forming an outerlayer of uniformly fine crystalline silica such as cristobalite ortridymite when heated to a temperature at which crystalline silica formscontaining crystallization promoting nuclei having a rate of diffusionin quartz glass less than that of sodium at elevated temperatures. Suchnuclei are preferably present in the outer half of the element wall.When the quartz glass element is exposed to elevated temperatures, thenuclei promotes the formation of the outer layer of uniformly finecrystalline silica which imparts thermal dimensional stability forextended periods of use at elevated temperatures.

RELATED APPLICATIONS This application is acontinuation-impart ofcopending applications Ser. No. 793,775, filed Jan. 24, 1969 and Ser.No. 810,713, filed Mar. 26, 1969, both now abandoned.

BACKGROUND This invention relates to quartz glass elements havingimproved thermal dimensional stability making them suitable for use atelevated temperatures for extended periods of time. More particularly,this invention relates to improved quartz glass elements having a hollowor tubular shape especially useful for producing semiconductor elementsat temperatures higher than previously attainable with prior quartzglass elements.

In the art of producing semiconductor elements such as diodes,four-layer diodes, transistors, integrated circuits and the like,diffusion processes are employed, for example, for doping thesemiconductors. For this purpose, the semiconducting crystal issubjected at high temperatures to the action of different gaseousatmospheres, for example, a phosphorous atmosphere and/or a galliumatmosphere. The semiconducting crystal or crystals which in many caseshave the shape of small plates are then secured on a carrier of quartzglass. For carrying out the diffusion process, this carrier upon whichthe semiconducting crystals are applied is inserted into a diffusiontube of quartz glass in an electrically heated annealing furnace. Thegaseous atmosphere which is predetermined for doping the semiconductingcrystals is then maintained in the diffusion tube at a predetermineddiffusion temperature. This is usually done by conducting the dopingsubstance through the quartz-glass tube in the form of a gaseouscurrent.

In order to carry out their functions properly, the semiconductingelements must possess certain properties, especially insofar as thecross-sectional diffusing shape, the resistance values and thedurability of the carriers are concerned. These properties are, however,determined 3,776,869 Patented Dec. 4, 1973 to a very considerable extentbythe amount of impurities and also byv the presence of so-calledsemiconductor poisons which might enter and contaminate thesemiconducting elements during the course of their production.Therefore, not only the raw material but also all other materials whichmight possibly affect the purity of these elements in the course oftheir production either directly or indirectly have to comply withextremely high requirements of purity. When quartz glass is employed asa material in the production of semiconducting elements, theserequirements previously necessitated the carriers for the semiconductingcrystals and the diffusion tubes to be made of a quartz glass of such ahigh degree of purity that it preferably contained a total of less than4 p.p.m. (parts per million) of metallic impurities.

In order for the diffusion treatment to be carried out within theshortest possible length of time, it should be effected at the highestpossible temperature since the rate of speed of diffusion increases veryrapidly as the temperature is increased. However, the upper temperaturelimit at which diffusion treatments could previously be carried out inactual practice amounted only to approximately 1,200 to 1,280" C. andoften they had to be carried out at a temperature of less than 1,200 C.since by remaining continuously in the annealing furnace the diffusiontubes of quartz glass were often plastically deformed to the extent thatthe carriers with the semiconducting crystals thereon would no longerfit into the tubes.

The French Pat. No. 1,293,554 discloses a diffusion tube which consistsof quartz and is provided on its outer side with a coating which willbecome liquid at the temperature at which the semiconducting crystalsare treated. This coating is intended to prevent impurities frompenetrating by diffusion through the quartz diffusion tube into the areawithin the quartz tube which forms the treating chamber for thesemiconducting crystals. However, such diffusion tubes of quartz glasswhich remain continuously in the annealing furnace also have thedisadvantage that they will be plastically deformed very lclonsiderablywhen the diffusion temperature is made tooigh.

The deterioration of diffusion tubes by cracking is caused by irregulardevitrification or recrystallization of the quartz glass brought on byexposing such tubes to elevated temperatures such as those encounteredin preparing semiconductor elements. Prior attempts to overcome thisproblem of thermal instability were directed at preventing or retardingdevitrification or recrystallization. For example, in U.S. Pat. No.2,904,713, quartz glass is produced wherein substantially nocrystallization seeds are present thus imparting to the quartz glass ahigh resistance against recrystallization. In U.S. Pats. Nos. 3,370,921and 3,472,667, elemental silicon or boron are utilized to create anoxygen deficiency in quartz bodies thus minimizing crystalline growth.And in U.S. Pat. No. 2,568,459, a glaze applied to the quartz surfaceretards and largely prevents quartz devitrification by preventing thediffusion of hydrogen through the hot quartz.

In the field of glass ceramics, increased resistance to breaking,cracking or failing due to mechanical impacts has been achievedaccording to U.S. Pats. No. 2,998,675 and 3,275,493 by combining silica,alumina and lithium oxide or magnesium oxide, in certain criticalproportions, with a metal oxide crystallization catalyst. Such glassceramic compositions are subjected to a heat treatment which results ina glass ceramic article having on its surface a thin, semicrystallinelayer, which because it has a linear thermal expansion coefficientsubstantially lower than the interior glass, establishes a compressivestress in and parallel to the surface after the article is cooled. Inother words, the interior glass shrinks more on cooling which tends tocompress the surface layer in effect mak- SUMMARY The present inventionprovides an improved quartz glass element, such as a diffusion tubeuseful in the production of semiconductor elements, capable of formingan outer layer of uniformly fine crystalline silica such as cristobaliteor tridymite when heated to a temperature at which such crystallinesilica forms which contain crystallization promoting nuclei having arate of diffusion in quartz glass less than that of sodium at elevatedtemperatures. Such nuclei are preferably present in the outer half ofthe element wall. When the quartz glass element is exposed to elevatedtemperatures, the nuclei promotes the formation of the outer layer ofuniformly fine crystalline silica which imparts to the element thermaldimensional stability for extended periods of use at elevatedtemperatures.

In an alternate embodiment, a quartz glass element, with or withoutcrystallization promoting nuclei as described above, is provided with athin outer layer of uniformly fine crystalline silica such ascristobalite or tridymite.

The quartz glass elements of the present invention are utilized intubular form in an improved diffusion process for preparingsemiconductor elements at temperatures of 1,200 C. and higher andpreferably at temperatures of 1,300 C. and higher.

THE DRAWINGS FIG. 1a shows a preferred embodiment of a quartz glass tube1 containing substantially uniformly distributed crystallizationpromoting nuclei 2 in the outer portion thereof on a greatly enlargerscale for illustrative purposes onl E IG. 1b is the same as FIG. 1a butillustrates the formation of a uniformly fine crystalline silica layer 3on a greatly exaggerated scale.

FIG. 2 shows an alternate embodiment of a quartz glass tube 10 providedwith a partial coating of crystalline silica 12.

FIG. 3 is a cross-sectional view taken along line II-II of FIG. 2.

FIG. 4 is a cross-sectional view of the coated tube of FIGS. 2 and 3provided with a protective coating 13.

FIG. 5 is a graph comparing the contraction of quartz glass with that ofcristobalite on cooling down from 1,l00 C.

DESCRIPTION It is an object of the present invention to provide aquartz-glass element, for example, of a tubular shape, which is designedso that, when employed in the production of semiconducting elementswhich is carried out at a high temperature, that is, of elements whichis carried out at a high temperature, that is, of elements such asdiodes, four-layer diodes, transistors, integrated circuits or the like,no impurities or semi-conductor poisons can pass from this elementeither directly or indirectly into the semiconducting crystals, andwhich also permits diffusion treatments to be carried out withoutdetrimental effects at still higher temperatures than could previouslybe employed.

It has. now been unexpectedly discovered that this object may beattained if the outr surface layer of the quartzglass element such as atube 1 shown in FIG. 1a contains not only silica but also at least oneadditional crystallization promoting substance 2 which has a degree ofconcentration of more than 4 ppm. and up to several hundred p.p.m.,preferably to 800 ppm, and does not form a poison which will affectthe'semiconducting element which is to be treated at a temperature above1,200" C. and preferably above 1,280 C., and which at temperaturesbetween approximately 1,200 C. and 1,380 C. has a low rate of speed ofdiffusion into silica as compared with the rate of sodium.

Prior to this invention it was conventional in the methods of producingsemiconductors at high temperatures to employ quartz glass of thehighest degree of purity as the material for making the diffusion tubeor, for example, the carrier of the semiconducting crystals. The quartzglass elements according to the invention in contrast contain additionalsubstances of particular types and of particular quantities. As anentirely unexpected and very important advantage of these quartz glasselements it has been found that they will show practically nodeformation even though they are maintained fora considerable length oftime at temperatures of approximately 1,300 C. Thus, it has especiallybeen found that, while diffusion tubes which are made in theconventional manner of quartz glass of a high degree of purity are oftenconsiderably deformed, those which are made according to the inventionso that their outer surface layer consists of silica and an additionalsubstance were not deformed. The quartz-glass elements according to theinvention therefore permit, for example, the diffusion treatment ofsemiconducting crystals to be carried out at considerably highertemperatures than could previously be employed and without danger thatthis might cause any detrimental secondary effects. The use of such hightemperatures permits the length of time of the diffusion treatment to bereduced considerably since the rate of speed of diffusion increasesexponentially with the temperature.

If the entire volume of the quartz-glass elements according to theinvention contains crystallization promoting nuclei aside from silica atthe rate of concentration as stated, they may be produced, for example,in a very simple manner from a homogenized melted mass consisting ofsilica and the desired nuclei. This technique is illustrated in Example1 herein. 1

For producing quartz-glass elements and especially the preferred tubularelements according to the invention in which only the outer surfacelayer of each element contains crystallization promoting nuclei asidefrom silica, it is possible to employ several different methods. Thus,for example, the surface of a tubular element of quartzglass of a highdegree of purity may be covered with the additional substance containingsuch nuclei, for example, by spraying or vaporizing it thereon,preferably, under a vacuum, and by then subjecting the coated tube'to' aheating process. An enamel coating and heating process for making suchtubular element is illustrated in Example 2 herein.

When the elements of the invention are employed, as for example whenthey are used as diffusion tubesin the semiconductor art, arecrystallization commences at temperatures ab'ove 1,000 C. with theformation of cristobalite and/or tridymite. This recrystallizationstarts quickly, uniformly and homogeneously, but on account of thedistribution of the nuclei of the invention it progresses slowly andsteadily. Crystal anisotropy, which may have the effect of diminishingstrength, has not been observed in the quartz glass parts of theinvention. The improved stability of shape at high temperatures which isdisplayed by quartz glass apparatus parts made bythe invention isprobably to be attributed to the constant growth of the crystallinesilica layer owing to the distribution of nuclei according to theinvention, and to the shrinkage and tension effects associatedtherewith. Lastly, an additional advantage is that due to thecompression of the quartz glass structure which this produces during theheating thereof, as .for example during the doping of semiconductorelements, the penetration of foreign ions which" interfere with thedoping process is virtually excluded.

According to another embodiment of the invention, the outside of atleast that part of a tube of quartz glass which is to be exposed to atemperature of more than l,000 C. is provided with a coating consistingof a cohesive finely crystalline silica layer which is composed ofcrystalline modifications of quartz, preferably a cristobalite layer,and has a thickness of less than 5% and preferably less than 1% of thethickness 'of the wall of the quartz-glass element or tube to which thiscoating is applied.

It was, a surprising fact that the quartz-glass elements or tubesaccording to the invention which were provided with a very thincohesive, finely crystalline outer layer or coating, especially ofcristobalite, were found not to break when a temperature threshhold ofapproximately 300 C. was surpassed either from below or from above,despite the fact which as such is well known in the art that, because ofa change in structure, the coeflicient of thermal expansion, forexample, of cristobalite possesses a point of unsteadiness within thistemperature range. This is probably due to the fact that the coatinglayer of finely crystalline cristobalite is extremely thin.

The quartz-glass tubes according to the invention which are providedwith such a cohesive, uniformly fine crystalline silica layer orcoating, especially of cristobalite, have also not shown anyconsiderable deformation when remaining for a longer period, forexample, of several weeks, under temperatures, which amounted up toapproximately l,300 C. The excellent mechanical stability of thesequartz-glass tubes also at temperatures of more than 1,000 C. isprobably due to the fact that the finely crystalline coating or layer ofthese tubes specially of cristobalite, is cohesive and that the crystalsstill grow at high temperatures.

Another feature of the invention which has proved to be of greatadvantage involves applying a protective coating over the finelycrystalline layer or coating. Such a protective coating preventsimpurities from penetrating into the quartz-glass tube, for example,from the muffle of the annealing furnace, during the period in which thequartz-glass tube is heated up to the temperature at which the doping ofthe semiconductors is carried out. Such impurities might result in theoccurrence of undesirable crystallizing processes in the quartz-glasstube. It has also been found that for producing such a protective layerit is of advantage to employ materials which at temperatures ofapproximately 1,300 C. do not evaporate very strongly but already becomeplastic or soft. Such materials may consist, for example, of germaniumoxide and silicon oxides or glass mixtures which are plastic at atemperature of approximately l,300 C.

As illustrated in FIG. 2, the quartz-glass tube is provided according tothe invention along a certain length thereof with a cohesive, finelycrystalline coating of crystalline silica 12 that is, preferably with acoating of cristobalite. Of course, it is also possible to provide theentire length of the quartz-glass tube 1 with such a finely crystallinecoating. It is, however, of advantage to apply this coating only uponthose parts of the quartz-glass tube which are exposed to hightemperatures of more than 1,000 C., such as the high temperaturesemiconductor doping zone. By coating only these parts of thequartzglass tube, the latter retains, for example, the possibility ofconnecting ground or unground quartz-glass elements to the uncoatedparts of the tube and especially to the ends thereof.

FIG. 4 illustrates a cross section of a quartz-glass tube 10 whichaccording to the invention is not only provided with a coating 12 offinely crystalline silica especially of cristobalite, but also with aprotective coating 13 which is applied upon the outer surface of thecoating 12.

The production of quartz-glass tubes which are provided with a coatingof cristobalite may be carried out, for example, by sprayingcristobalite powder of high purity upon the quartz-glass tube and byburning this coating into the outer surface of the quartz glass by meansof a flame or in a furnace and, if desired, by maintaining thequartz-glass tube at a high temperature for such a length of time untilthe burned-in nuclei have grown together into a cohesive, finelycrystalline layer as distinguished from a distinct overlying-coating. Ifthe area of the quartz-glass tube upon which the cristobalite coating isapplied has, for example, a wall thickness of approxi-v mately 2 mm.,the thickness of the layer of cristobalite amounts, for example, to lessthan 0.02 mm.

The phrase crystallization promoting nuclei is intended to include thosesubstrances that vw'll promote the formation of crystalline silica suchas cristobalite and tridymite for example when a quartz-glass element ofthe invention is heated to a temperature at which such crystallinesilica forms.

For purposes of this invention and particularly in the case of diffusiontubes, the temperature at which crystalline silica forms falls withinthe range of 900 to 1,S50 C. preferably in the range of 900 C. to l,350C. Crystalline silica is known to have several forms among which arealpha and beta quartz, alpha and beta cristobalite and alpha and betatridymite.- In the temperature range referred to above, the formation ofcertain crystalline forms is favored and two or more crystalline formsmay be present at the same time. For example, in the temperature rangesindicated it is believed that beta cristobalite forms faster than otherforms of crystalline silica and for this reason is probably thepredominate type of crystalline silica formed according to thoseinvention. However, this invention is in no way limited or restricted tothe formation of any particular form or forms of crystalline silica. Itis sufficient to form a layer of crystalline silica (Whatever its formor forms) as shown in FIG. 1b by reference number 3 to obtain the highlydesirable properties and advantages of this invention.

It should also be understood that crystalline silica, regardless of typeor types, is converted to amorphous or vitreous silica when heated toits fusion temperature which is about 1,730" C., preferably above about1,750 C., for a period of time suflicient to complete the conversion.Vitreous silica once formed by fusion remains in this form when cooleddown to ambient temperatures. It is only when an article formed fromvitreous silica is reheated does devitrification or recrystallizationoccur. The amorphous form of silica is also known as quartz glass, fusedsilica, vitreous silica, vitreous quartz and fused quartz. Regardless ofthe nomenclature, the important difference is that quartz glass elementsof the invention are amorphous at the outset and the nuclei presenttherein forms a layer of crystalline silica when the element is exposedto temperatures in the range of 900 C. to 1,550 C.

Thus, the formation of quartz-glass elements containing nuclei can becarried out without forming crystalline silica due to the fact that attemperatures above the fusion temperature of silica, about 1,730 C.,only amorphous silica is formed. Thus, it is possible to supplysemiconductor manufacturers with nuclei containing quartzglass tubesafter which the beneficial crystalline silica layer is formed during thediflusion process.

The crystallization promoting nuclei are atomic in dimension and includeone or more atoms, ions and/or individual molecules and are the centersaround which the crystalline silica forms and grows according to theinvention. Suitable nuclei have a rate of diffusion in quartzglass lessthan that of sodium at elevated temperatures, for example l,000 C. orhigher, and include an element from Group IV of the periodic table,boron, aluminum, phosphorus, antimony, zinc, mangesium, calcium, galliumand mixtures thereof. Compounds from which nuclei are obtained includethe oxides, carbides or nitrides of any of the foregoing, as for examplealuminum nitride, aluminum oxide, germanium oxide, tin oxide, siliconcarbide, silicon nitride, silica suboxides, and the like. Zinc,magnesium, calcium, tin, boron, phosphorus, aluminum, titanium,zirconium hafnium, antimony and gallium ions because of their large ionradius have been found to be especially useful for forming finecrystalline silica layerers. Most preferred is aluminum.

According to the present present invention, crystallization promotingnuclei are incorporated into the molecular structure of the quartzglass. Methods for accomplishing this are illustrated in the examples.For instance, in Example 1, a solution of a nuclei compound is coated onquartz powder which is subsequently fused into quartz glass. The fusionstep also decomposes the nuclei compound leaving only the nuclei such asone or more metal ions which are entrapped and incorporated into themoleuclar structure of the quartz glass. This quartz glass containingnuclei is then drawn into a tube using conventional techniques which canbe used directly or slipped over and fused with a pure quartz glass tuberesulting in a tube as shown in FIG. la. In Example 2, the quartz glasscontaining nuclei is ground and added to an enamel composition which iscoated onto and fused with a pure quartz glass tube again resulting in atube as shown in FIG. 1a.

During the first four to twenty hours of use of a tube according to theinvention in a semiconductor doping process, it is believed that auniformly fine crystalline silica layer forms to a depth approximatelyequal to the depth of the nuclei. Further exposure to elevatedtemperatures causes the nuclei to diffuse or migrate further into thetube wall and the crystalline silica layer to continue to growinherently. However, once the uniformly fine crystalline layer isformed, nuclei diffusion and inherent crystalline growth proceeduniformly and slowly resulting in greatly improved tube life and theability to use higher temperatures than could be used with prior quartzglass diffusion tubes.

The nuclei used in this invention are present in an amount of form about4 to about 800 p.p.m .Uniform distribution of the nuclei is preferred inorder to form uniformly fine crystalline silica layers. Concentrationsof nuclei ranging between about 10 to 800 p.p.m. are especiallypreferred.

The concentrations of nuclei at the outer surface of the quartz-glasselement is generally less than about -10 nuclei per sq. cm. andpreferably in the range of about 1-10 to about 5-10 nuclei per sq. cm.Stated in different terms, the surface concentration of nuclei is in therange of about 5-10- to about 25 micrograms per sq.

In the present invention, rather than attempt to prevent or retarddivitrification of quartz glass as taught by the prior art, theformation of crystalline silica is promoted in a uniform, very finemanner to yield modified quartz glass elements having totally unexpectedlonger use at temperatures higher than those that could previously beemployed, e.g., l,300 C. and higher.

Also, as illustrated by the graph which comprises FIG. 5, thecoeflicient of thermal expansion and contraction of crystalline silicasuch as cristobalite is appreciably greater than quartz glass. Thus,when a crystalline silica layer is formed in a quartz glass element atelevated temperature and cooled down, the interior quartz glass shrinksless than the outer crystalline layer tending to stretch same therebyestablishing a tensile stress in and parallel to the outer crystallinelayer. The mechanical stability (surface hardness) of an articleaccording to the invention is less at room temperature than themechanical stability of customary quartz glass also at room temperature.With respect to the known glass ceramic articles having a compressivestress layer, the object is to achieve above all a high mechanicalstability at room temperature, whereas their dimensional stability atelevated temperature is reduced, As against that, the object of theinvention is to produce articles having a high dimensional stability atroom temperature is actually reduced.

Thus, the present invention in its preferred embodiment of a diffusiontube for producing semiconductor elements may have crystallizationpromoting nuclei uniformly dispersed throughout only the outer portionof the tube to a depth no greater than one-half of the tube wallthickness. In addition, the outer surface of a quartz glass tube, withor without nuclei as described herein, may be provided with a coating ofvery pure crystalline silica powder such as cristobalite and then heatedto form a crystalline silica coating on or a layer within the quartzglass tube. In the instance where crystalline silica powder is coatedonto a tube containing nuclei, such nuclei promote the further formationof a layer of crystalline silica in the tube. Because applying aseparate coating of crystalline silica involves powder coatingtechniques and obtaining good adhesion, it is preferred to employ tubesof quartz glass containing only nuclei as described herein andpermitting the beneficial crystalline silica to form upon exposure toheat in the diffusion process, for example. I

In prior quartz glass elements, impurities (such as those sought to beremoved in U.S. Pat. 2,904,713) are believed to be the seeds fordevitrification which, once initiated proceeds nonuniformly anduncontrolled until the entire element is devitrified. Thisdevitrification known for diffusion tubes of luminous-discharge lampsand which results from the accidental and nonuniform distribution ofimpurities leads to a nonuniform devitrification of the diffusion tubecreating nonuniformly distributed stresses in the tube resulting in thebreakage of the tube. In the present invention, the devitrificationprocess is utilized in a unique way resulting in higher use temperaturesover longer periods of time than theretofore available. The presence ofnuclei or a crystalline silica coating as described herein causesdevitrification to occur but with the formation of a uniform, very finecrystalline silica layer which, because of its uniform and fine nature,results in very slow progressive devitrification. This slowdown ofdevitrification has resulted in a 25 to 50 percent increase in theuseful life of quartz glass diffusion tubes.

The following examples are intended to illustrate the present inventionwithout limiting the same in any manner.

EXAMPLE 1 Fabricating procedure for a diffusion tube with uniformlydispersed crystallization promoting nuclei throughout the outer portionof the wall of the tube.

Pure quartz crystal granules (e.g. 10 kgs.) as they are customarily usedfor the fusion of pure quartz glass, are wetted by pouring over them aconcentrated aqueous solution of aluminum nitrate which contains 8 g. ofAl-ions, and subsequently dried with continuous stirring. The resultingquartz glass granules have the Al salt adhering to them and have anAl-ion content of 1,200 p.p.m. These granules are placed into a graphitecrucible and heated in a vacuum melting furnace steadily and slowly (for3 to 4 hours) to a melting temperature (approx. 1,750 C.) and, bysubsequently maintaining the melting temperature during 30 to 60minutes, fused into clearly transparent quartz glass. The vacuum meltingfurnace is an electrically heated (inductively or by current passage)furnace within a vacuum chamber which, prior to and during the meltingprocedure, is evacuated to a pressure of approx. 10" torr by mechanicalvacuum pumps. The resulting fused quartz glass block contains 200 to 800p.p.m. of Al, and is subsequently drawn to a tube in an electricallyheated quartz glass drawing furnace. The tube drawing furnaceessentially consists of a graphite crucible wherein the quartz glassblock is lying; at its bot-tom the crucible has an opening with a nozzleand a mandrel forming the out side and the inside surface of the drawntube during the drawing process. The electrical heating elements arearrang d. in the same way as in the melting furnace. However, in thetube drawing, no evacuation takes place. During drawing the drawnAl-containing tube is dimensioned in such a way that it can becomfortably slid over a quartz glass tube without additives and is thenfused together with same on a glass blowers lathe to form one tube unit.Basically, this heating process (fusing together of inside and outsidetubes) can also take place on a tube drawing bench, at the same timereducing the fused tube portion to a narrower tube. The thickness of thecrystallization promoting nuclei containing outer portion of the wall ofthe tube is obtained from the purely geometric wall thickness ratiobetween the inner pure quartz glass tube and the outer Al-containingquartz glass tube.

EXAMPLE 2 Another method for producing tubes according to the inventionhaving a predominantly thin outer wall portion with respect to theentire wall is to grind Al-containing quartz glass made according toExample 1 into quartz glass powder (1-20 grain size). Thereupon anenamellike paste or suspension is made from this powder according to thefollowing formula:

1,000 g. of the ground quartz glass powder is mixed with 500 g. ofbi-distilled water to which was previously added,

1 g. of dextrine-adhesive. In order to make this suspension more stable,

1-20 g. of pure molecular-dispersed silicic acid may be added.

This suspension is then applied to the outer surface of a pure quartzglass cylinder (approx. 4-6 g./dm. and dried. Thereupon this cylinder isdrawn into a tube according to the invention on a tube drawing bench asdescribed in Example 1.

EXAMPLE 3 The production of stabilized tubes according to the inventionhaving an outer protective layer is achieved by applying on a stabilizedtube a known, more readily fusible glass composition in the form of apowder or in the form of a solution in accordance with known enamellingtechniques or in the form of a thin glass film in accordance with knownglass processing techniques (e.g. in the form of a thin-walled tube) andfusing into a tube unit in a further heating process.

This process can also be used for tubes prepared according to Example 2fusing together in the subsequent heating process the protective coatingas well as the sta- 10 bilizing coating with the pure quartz glass tubeor hollow cylinder.

What is claimed is:

1. Quartz glass element capable of forming a tensilely stressed outerlayer of uniformly fine crystalline silica when heated to a temperatureat which such crystalline silica forms containing in the outer surfacelayer of said element from about 4 to about 800 p.p.m. of crystalizationpromoting nuclei having a rate of diffusion in quartz glass less thanthat of sodium at elevated temperatures.

2. Element of claim 1 wherein said nuclei are present in a concentrationof from about 10 to about 800 p.p.m.

3. Element of claim 1 wherein said nuclei are selected from the group ofa Group IV element, boron, aluminum, phosphorus, antimony, zinc,magnesium, calcium, gallium, and mixtures of the foregoing.

4. Element of claim 1 wherein said element is a diffusion tube forproducing semiconductor elements.

5. Diffusion tube of claim 4 wherein said nuclei are present in aconcentration of from about 10 to about 800 p.p.m.

6. Element of claim 1 wherein the surface concentra tion of said nucleiis less than about 5-10 nuclei per sq. cm.

7. Element of claim 4 wherein the surface concentration of said nucleiis from about 1-10 to about 5-10 nuclei per sq. cm.

8. Element of claim 1 having said tensilely stressed outer layer formedby heating to a temperature at which crystalline silica forms.

9. Diffusion tube of claim 4 having said tensilely stressed outer layerformed by heating to a temperature at which crystalline silica forms.

10. Element of claim 1 wherein said nuclei are uniformly distributed.

References Cited UNITED STATES PATENTS 2,998,675 9/1961 Olcott et al49-77 3,275,493 9/1966 MacDowell 161-1 3,445,252 5/1969 MacDoWell l06393,298,553 1/1967 Lusher 215-1 3,116,137 12/1963 Vasilos et al. 6-18DANIEL J. FRITSCH, Primary Examiner US. Cl. X.R. -33, Dig. 8; l0639 DV,47, 52; 138-141, 145, 177; 1611, 192, 193

